The hitherto-known processes for producing vitamin D.sub.3 derivatives having a hydroxy group in the 1.alpha.-position include, for example, the process for producing 1.alpha.-hydroxy-vitamin D.sub.3 using cholesterol as a starting material (Japanese Patent Application Laid-open No. 48-62750 and No. 49-95956), the process for producing 1.alpha.,25-dihydroxy-vitamin D.sub.3 using cholesta-1,5,7-trien-3-on-25-ol derived from cholesta-1,4,6-trien-3-on-25-ol as a synthetic intermediate (Japanese Patent Application Laid-open No. 51-100056), and the process for producing (24R)-1.alpha.,24,25-trihydroxy-vitamin D.sub.3 by subjecting (24R)-1.alpha.,3.beta.,24,25-tetrahydroxycholesta-5,7-diene to ultraviolet irradiation in an inert organic solvent and, then, isomerizing the resulting (24R)-1.alpha., 24,25-trihydroxy-previtamin D.sub.3 (Japanese Patent Application Laid-open No. 51-108046). It is also known that certain pregnane derivatives, such as (20S)-1.alpha.,3.beta.-diacetoxypregn-5-ene-20-carbaldehyde, etc., which are derived from (20S)-21-hydroxy-20-methyl-6.beta.-methoxy-3.alpha.,5-cyclo-5.alpha.-pregn ane, (20S)-21-hydroxy-20-methylpregna-1,4-dien-3-one, etc. by the processes shown below can be converted to 1.alpha.,25-dihydroxy-vitamin D.sub.3 and other compounds (Japanese Patent Application Laid-open No. 53-50152). ##STR2## In the above formulas, THP is a 2-tetrahydropyranyl group; Ac is an acetyl group; the dotted line ( . . . ) indicates that the substituent is in the .alpha.-configuration; the solid line (-) indicates that the substituent is in the .beta.-configuration; the wavy line (.about.) indicates that the substituent is either in the .alpha.-configuration or in the .beta.-configuration.
There is also known a process by which 1.alpha.,25,26-trihydroxy-vitamin D.sub.3 is synthesized from the aforementioned (20S)-1.alpha.,3.beta.-diacetoxypregn-5-ene-20-carbaldehyde (Japanese Patent Application Laid-open No. 56-51447).
While a variety of processes are thus known for the production of vitamin D.sub.3 derivatives having a hydroxy group in the 1.alpha.-position, it is obviously preferable that, in the production of such vitamin D.sub.3 derivatives having a hydroxy group in the 1.alpha.-position, one have as many compounds as possible to choose from as synthetic intermediates, for one would then enjoy the utmost flexibility in selecting the optimum process sequence according to the availability of starting compounds.
It is, therefore, an object of the invention to provide novel compounds which can be converted to various vitamin D.sub.3 derivatives having a hydroxy group in the 1.alpha.-position.
It is another object of the invention to provide processes for producing such novel compounds.